Polyester has long been recognized as a desirable material for textile applications. The basic processes for the manufacture of polyester are relatively well known and straightforward, and fibers from polyester can be appropriately woven or knitted to form textile fabric. Polyester fibers can be blended with other fibers such as wool or cotton to produce fabrics which have the enhanced strength, durability and memory aspects of polyester, while retaining many of the desired qualities of the natural fiber with which the polyester is blended.
As with any fiber, the particular polyester fiber from which any given fabric is formed must have properties suitable for manufacture, finishing, and end use of that fabric. Typical applications include both ring and open-end spinning, either with or without a blended natural fiber, weaving or knitting, dyeing, and finishing. In addition, it has long been known that synthetic fibers such as polyester which are initially formed as extruded linear filaments, will exhibit more of the properties of natural fibers such as wool or cotton if they are treated in some manner which changes the linear filament into some other shape. Such treatments are referred to generally as texturizing, and can include false twisting, crimping, and certain chemical treatments.
In a homopolymeric state, polyester exhibits good strength characteristics. Typical measured characteristics include tenacity, which is generally expressed as the grams per denier required to break a filament, and the modulus, which refers to the filament strength at a specified elongation ("SASE"). Tenacity and modulus are also referred to together as the tensile characteristics or "tensiles" of a given fiber. In relatively pure homopolymeric polyester, the tenacity will generally range from about 3.5 to about 8 grams per denier, but the majority of polyester has a tenacity of 6 or more grams per denier. Only about 5 percent of polyester is made with a tenacity of 4.0 or less.
In many applications, of course, it is desirable that the textile fabric be available in a variety of colors, accomplished by a dyeing step. Substantially pure polyester, however, is not as dyeable as most natural fibers, or as would otherwise be desired, and therefore must usually be dyed under conditions of high temperature, high pressure, or both, or at atmospheric conditions with or without the use of swelling agents commonly referred to as "carriers". Accordingly, various techniques have been developed for enhancing the dyeability of polyester.
One technique for enhancing the dyeability of polyester is the addition of various functional groups to the polymer to which dye molecules or particles such as pigments themselves attach more readily, either chemically or physically, depending upon the type of dyeing technique employed. Common types of additives include molecules with functional groups that tend to be more receptive to chemical reaction with dye molecules than is polyester. These often include carboxylic acids (particularly dicarboxylic or other multifunctional acids), and organo metallic sulfate or sulfonate compounds.
Another additive that has been proposed is polyethylene glycol ("PEG"), which has been shown to offer advantages when incorporated with polyester into textile fibers, including antistatic properties and improved dyeing characteristics. If other practical factors and necessities are ignored, adding increased amounts of PEG to polyester will increase the dyeability of the resulting polymer. Nevertheless, there are a number of disadvantages associated with the application of polyethylene glycol to polyester using these prior techniques, particularly when the PEG is added in amounts of 5 to 6 percent or more by weight, amounts which the prior references indicate are necessary to obtain the desired enhanced dyeability. These disadvantages are not generally admitted in the prior art patents and literature, but are demonstrated to exist by the lack of known commercial textile processes which use fibers formed essentially solely from copolymers of polyester and polyethylene glycol. These shortcomings can be demonstrated, however, by those of ordinary skill in the art using appropriate evaluation of the prior technology.
Most notably, commercially available fibers formed from polyester-polyethylene glycol copolymers tend to exhibit improved dyeability at the expense of tensiles; improved dyeability at the expense of shrinkage; improved tensiles at the expense of shrinkage; poor light fastness; poor polymer color (whiteness and blueness); unfavorable process economies; and poor thermal stability.
In some earlier techniques, in addition to the negative characteristics introduced into polyester fiber by the addition of polyethylene glycol, it has been believed that where amounts smaller than 5 to 6 percent of polyethylene glycol are used, they must be used in conjunction with some other molecule or functional group which would concurrently enhance the dyeability of the fiber. For example, U.S. Pat. No. 4,049,621 issued to Gilkey et al states that polyester fibers enhanced with less than 6 weight percent polyethylene glycol do not exhibit acceptable dyeability without a carrier. None of the prior techniques teach or suggest that modification of polyester fiber with polyethylene glycol alone in amounts lower than about 5 percent can have any significant beneficial effect on the various desirable characteristics of a polyester fiber.
Occasionally polyethylene glycol has been used in the manufacture of polyester fiber in conjunction with other additives to compensate for the disadvantages introduced by those other additives. For example, in U.S. Pat. No. 4,526,738 issued to Miyoshi et al, a metal sulfoisophthalic group is added to permit the dyeability of polyester fiber with cationic or basic dyes. This functional group, however, suppresses the melting point, lowers the tenacity, and increases the melt viscosity of the resulting polyester and fiber formed therefrom. In order to compensate for these disadvantages, polyethylene glycol is added to moderate both the suppression of the melting point and the increase in melt viscosity of the polyester while still encouraging increased dyeability. As noted by Miyoshi, however, the resulting polymer must be maintained under rather specific conditions of degree of polymerization.
Accordingly, there exists no commercially viable method for using polyethylene glycol alone to enhance the dyeing properties of polyester fiber without sacrificing desirable characteristics of strength, shrinkage, light fastness, thermal stability and color.